Normally the two coronary arteries arise from the aorta: one, the right (R), arises from its right or anterior side and continues as a single artery; and the second, the left main (LM), arises from its left or posterior side and subdivides into two arteries, the left anterior descending (LAD) and the left circumflex (LC) coronary arteries. Severe narrowing of the LM may lead to inadequate oxygenation of left ventricular (LV) myocardium perfused by the LAD and LC. Rarely, the main coronary artery which subdivides into two major branches arises from the right side of the aorta (right main [RM]) rather than from the left side, and when this circumstance exists the main trunk (RM) subdivides either into the R and LC or into the R and LAD (conus branch). Severe narrowing of the RM in this circumstance is equivalent to severe narrowing of the LM but the LV myocardium deprived or potentially deprived of adequate oxygenation would be that supplied by the R and LC or the R and LAD, not that supplied by the LAD and LC. We studied at necropsy the hearts of two patients with fatal atherosclerotic coronary heart disease in whom the main or subdividing coronary artery arose from the right side of the aorta, rather than from the left side, yielding a RM rather than an LM coronary artery. In one patient, a 35-year-old man who died suddenly 42 days after an acute myocardial infarction, the LC and R arose from the RM and the RM was severely narrowed by an atherosclerotic plaque. In the second patients a 60-year-old man who died shortly after aortocoronary bypass grafting for severe angina pectoris, the LAD and R arose from the RM which also was severely narrowed by atherosclerotic plaque, as were all major coronary arteries.